This invention relates to heart pacemakers in general and more particularly to an improved heart pacemaker using a biofuel cell as the energy supply.
The use of biofuel cells to supply energy or power to a heart pacemaker, the biofuel cell being implanted in the body, is known in the art. In particular, a glucose/oxygen biofuel cell is disclosed in German Offenlegungsschrift No. 2,200,054 and U.S. Pat. No. 3,861,397, respectively. In such biofuel cells the chemical energy of the body substances is utilized to generate the electrical energy required to operate the pulse generator of a heart pacemaker. The pulse generator then stimulates the myocardium to beat by means of electrical pulses transmitted through a stimulating electrode. By using a biofuel cell as the energy supply of the heart pacemaker frequent replacement of the energy source is not necessary as is the case where primary cells such as Zn/HgO or Cd/HgO cells are used. As a result, relatively frequent operations for energy source replacement which are expensive, expose the patient to danger of infection and make demands on hospitals, clinics and physicians are avoided.
Typically the stimulating electrode disposed in the heart is a noble metal electrode with platinum being used in most cases. However, such electrodes generally establish a relatively high potential, e.g. approximately 800 to 900 mV, when measured with respect to an hydrogen electrode in the same electrolyte. This is because they also function as electrocatalysts whereby they assume the potential of an oxygen electrode. However this results in poor blood compatibility. Only electrodes which are negative or slightly positive have good blood compatibility. Relatively high positive potential such as the oxygen potential can lead to blood coagulation. Another disadvantage resulting from a high potential in the stimulating electrode is poor body compatibility. The stimulating electrode has a strong catalytic oxidizing effect on proteins, and in particular on the muscle tissue to which the stimulus is to be transmitted. As a result, the tissue adjacent to the stimulating electrode suffers irreversible damage and is destroyed, i.e. connective tissue capsules are formed. Since the stimuli can only act on healthy tissue some distance away from the electrode a greater amount of energy must be supplied. Furthermore the current density drops off because of this effect since the stimulating electrode is generally of a hemispherical shape. Preferably the stimulation is carried out with a cathodic polarization of the stimulating electrode since the stimulation threshold is more favorably situated for a cathodic stimulating pulse than for an anodic stimulating pulse.
In view of the difficulties mentioned above, it is the object of the present invention to provide a pacemaker which has improved body and blood compatibility.